In typical dry-type electrophotographic processes, latent electrostatic images formed on a photoconductor are developed by means of a triboelectrically charged toner. Specifically, when an image is formed by a photographic copying machine, a surface of a roller composed of a photoconductive and photosensitive material is charged. An electrostatic latent image is formed by exposure to the light reflected from the surface of an original to be copied. The latent image is developed by a toner, with the formed visible image being transferred to a paper or the like. The transferred image is fixed on the paper by compression under heating, and thus a copy print is obtained.
A typical toner includes several components including a toner resin, a colorant and an electrostatic carrier material. Traditionally, toner resins have been made by compounding and melting the toner components (resin, colorant, etc.), followed by extruding the mixture into strands or pellets. These solid masses are then pulverized into small particles and sorted to provide the desired particle size and particle size distribution. While this method has been widely used, it has several limitations. For example, forming toner particles having a size less than about 8 micron is not economically feasible using conventional techniques. In addition, the shape of the toner particles may be undesirably non-uniform, which may affect toner properties, such as charge-to-mass ratio. Furthermore, narrow particle size distributions may be difficult to achieve. Other problems include the energy intensive pulverizing process and the inability to tailor the microstructure of the toner particles.
The disadvantages associated with conventional methods of producing toner have led to the so-called “chemically-produced toner” or CPT. As opposed to the “large-to-small” approach of conventional toner processes, CPT processes use a “small-to-large” approach, using polymerization techniques to form the basic toner particle structure. While there are many different types of CPT, typical polymerization techniques include suspension polymerization, emulsion polymerization and aggregation, microencapsulation, dispersion and condensation polymerization. These techniques can produce toner particles in a 3 to 5 micron size or less range, and may produce narrower particle size distributions than typically obtained from conventional processes. In addition, relatively uniform toner particle shape may be achieved and particle composition may be more precisely controlled. Examples of CPT include the toner resins discussed in U.S. Pat. No. 3,634,251, which describes CPT synthesized by a suspension polymerization process; and the toner resins discussed in U.S. Pat. No. 4,027,048, which describes a CPT microencapsulation process.
One disadvantage with current methods for producing CPT is that they require an emulsifier/surfactant. This may be problematic because a small amount of residual surfactant may undesirably affect the properties of the toner particles, including the tribocharge, and the removal of all of the surfactant from the particles may be difficult to achieve. Furthermore, these processes also often require a chain transfer agent, such as dodecanthiol, to control molecular weight. Residual chain transfer agent may also undesirably affect the toner properties.
Therefore, it would be desirable to obtain CPT that is not synthesized using a surfactant and/or a chain transfer agent. Such toner resins should ideally have desirable pigment dispersion, heat resistance and chargeability, as well as relatively low fusing temperatures.